Conventionally, there is a scheme for a base station to use an adaptive array antenna and improve transmission quality through transmission with directivity.
On the other hand, there is also a scheme called “adaptive modulation” in mobile communications under which a modulation/demodulation scheme is adaptively switched according to fluctuations in reception quality caused by fading. Setting an optimum modulation scheme according to a communication environment allows a large volume of packet data to be transmitted at high speed.
When introducing a technology of an adaptive array antenna to a system carrying out adaptive modulation, a communication terminal apparatus measures a reception SIR (ratio of a desired signal to interference signal) using a common pilot signal (CPICH) transmitted with directivity from a base station apparatus as a reference signal. Then, the communication terminal apparatus notifies the base station apparatus of a measured reception SIR. The base station apparatus switches a spreading factor, multi-code multiplexing number, data modulation multi-valued number and error correcting coding rate according to the notified reception SIR and sends packet data to the communication terminal apparatus.
In this case, the base station apparatus performs transmission/reception with a plurality of moving communication terminal apparatuses, and it is often the case that there is a plurality of communication terminal apparatuses requesting reception of high-speed and large-volume packet data over a wide range of area. In order to transmit packet data with directivity to all communication terminal apparatuses requesting reception of high-speed and large-volume packet data, the base station apparatus needs to transmit the packet data by switching between directivities. Therefore, a reference signal used to measure reception quality which is the basis for adaptively modulating packet data needs to be transmitted with directivity to obtain accurate reception quality information. Thus, when the adaptive array antenna technology is introduced to a system carrying out adaptive modulation, the directivity when a reference signal is transmitted and the directivity when packet data is transmitted are switched as appropriate.
However, in the case of a conventional base station apparatus and communication terminal apparatus, it may be necessary to often change directivity control when communication terminal apparatuses exist over a wide range of area and there may exist a time difference in transmission timing between a reference signal and packet data, with the result that although adaptive modulation, etc., is performed based on reception quality measured using the reference signal, the measured reception quality may not be reflected when packet data is actually transmitted with directivity. In this case, even if the modulation multi-valued number and coding rate, etc., are optimized and packet data is transmitted, there may be a situation in which the communication terminal apparatus cannot help but receive the packet data with quality much inferior to the expected quality, being unable to demodulate the packet data or there may also be a situation in which the quality becomes by far better than the expected quality, causing the efficiency of the radio frequency band to decrease, and even if the adaptive array antenna technology is introduced to the system carrying out adaptive modulation, it is not possible to make the most of advantages of both adaptive modulation and adaptive array antenna technology or maximize communication path utilization efficiency.